Bazedoxifene (1-[4-(2-azepan-1-yl-ethoxy)benzyl]-2-(4-hydroxyfenyl)-3-methyl-1H-indol-5-ol) of formula 1:
belongs to selective estrogen receptor modulators (SERMs). A more detailed description of its biological activity can be found e.g. in Drugs of the Future, 2002, 27(2), 117-121.The preparation of bazedoxifene and its acetate was published in the U.S. Pat. Nos. 5,998,402 and 6,479,535. The preparation of bazedoxifene was also published in J. Med. Chem. 2001, 44, 1654-1657. US 2005/0227965 A1 compares two crystalline polymorphs of bazedoxifene acetate. US 2005/0227964 A1 discloses bazedoxifene ascorbate and its use.
Use of a substance for pharmaceutical purposes places high demands on the substance quality. The most efficient purification operation is crystallization. In the case of substances that cannot crystallize it is very difficult to achieve internationally appreciated quality criteria defined by the ICH instructions. In addition, amorphous forms of substances are more susceptible to decomposition, in particular hydrolysis or oxidation, which is caused by their large surface. According to well-known and verified information bazedoxifene and its hitherto known forms can be classified among such substances that are difficult to purify and prone to decomposition.
Bazedoxifene as well as bazedoxifene ascorbate are non-crystalline forms that can be obtained by concentrating the solution until dry. Due to this, however, all undesired, non-volatile substances coming from the synthesis or decomposition of API remain in the resulting substance. This is mainly the case of 2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol of formula 2 and 1-[4-(2-azepan-1-yl-ethoxy)benzyl]-5-benzyloxy-2-(4-hydroxyphenyl)-3-methyl-1H-indole of formula 3, coming from the synthesis, and 1-(4-hydroxybenzyl)-2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol of formula 4 and 1-[4-(2-azepan-1-yl-ethoxy)benzyl]-2-(4-hydroxyphenyl)-3-methyl-1H-indol-5-ol N-oxide of formula 5, generated by decomposition of bazedoxifene.

The allegedly crystalline acetate does not solve the problem of quality. The presence of even small quantities of the above mentioned or other impurities precludes crystallization of bazedoxifene acetate prepared in accordance with the patent documents U.S. Pat. Nos. 5,998,402; 6,479,535 or US 2005/0227965. The acetate can be successfully obtained in the amorphous form by a modified procedure with yields of about 40%. The acetate is subject to decomposition reactions producing undesired impurities, especially oxidation that produces the undesired substance of formula 5. To suppress this decomposition process the addition of ascorbic acid as an antioxidant is commonly used. Therefore, a more suitable form of bazedoxifene has been looked for that could be easily prepared in high yields and in particular in the quality corresponding to API and would not be susceptible to undesired decompositions at the same time.